Quality control scheme for multiple-input multiple-output (mimo) orthogonal frequency division multiplexing (ofdm) systems

ABSTRACT

A method and apparatus for optimizing the system capacity of an Orthogonal Frequency Division Multiplexing (OFDM) system that uses with Multiple-Input Multiple-Output (MIMO) antennas. In a receiver, a target quality of service (QoS) metric and reference data rate are set. The target QoS metric may be set to a predetermined value and/or may be adjusted dynamically with respect to packet error rate (PER) by a slow outer-loop control processor. The QoS of received signals are compared to the target QoS. Depending on the comparison, the receiver generates a channel quality indicator (CQI) which is sent to the transmitter. The CQI is a one or two bit indicator which indicates to the transmitter to disable, adjust or maintain data transmission rates of particular sub-carriers, groups of sub-carriers per transmit antenna, or groups of sub-carriers across all transmit antennas. At the transmitter, the transmitted data rate is disabled, adjusted or maintained.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 11/118,867,filed on Apr. 29, 2005, which claims priority from ProvisionalApplication No. 60/598,183, which was filed on Aug. 2, 2004 which isincorporated by reference as if fully set forth.

FIELD OF INVENTION

The present invention relates to wireless communications. Moreparticularly, the present invention relates to a method and apparatusfor optimizing the system capacity of an Orthogonal Frequency DivisionMultiplexing (OFDM) system that uses with Multiple-Input Multiple-Output(MIMO) antennas.

BACKGROUND

Orthogonal Frequency Division Multiplexing (OFDM) is an efficient datatransmission scheme where the data is split into smaller streams andeach stream is transmitted using a sub-carrier with a smaller bandwidththan the total available transmission bandwidth. The efficiency of OFDMresults from selecting sub-carriers that are mathematically orthogonalto each other. This orthogonality prevents closely situated sub-carriersfrom interfering with each other while each is carrying a portion of thetotal user data.

For practical reasons, OFDM may be preferred over other transmissionschemes such as Code Division Multiple Access (CDMA). When user data issplit into streams carried by different sub-carriers, for example, theeffective data rate on each sub-carrier is less than the totaltransmitted data rate. As a result, the symbol duration of datatransmitted with an OFDM scheme is much larger than the symbol durationof data transmitted with other schemes. Larger symbol durations arepreferable as they can tolerate larger delay spreads. For instance, datathat is transmitted with large symbol duration is less affected bymulti-path than data that is transmitted with shorter symbol duration.Accordingly, OFDM symbols can overcome delay spreads that are typical inwireless communications without the use of a complicated receiver forrecovering from such multi-path delay.

Multiple-Input Multiple-Output (MIMO) refers to a type of wirelesstransmission and reception scheme wherein both a transmitter and areceiver employ more than one antenna. A MIMO system takes advantage ofthe spatial diversity or spatial multiplexing options created by thepresence of multiple antennas. In addition, a MIMO system improvessignal quality, such as for example signal-to-noise ratio (SNR), andincreases data throughput.

Multi-path, once considered a considerable burden to wirelesscommunications, can actually be utilized to improve the overallperformance of a wireless communication system. Since each multi-pathcomponent carries information about a transmitted signal, if properlyresolved and collected, these multi-path components reveal moreinformation about the transmitted signal, thus improving thecommunication.

Orthogonal Frequency Division Multiplexing (OFDM) systems that are usedwith Multiple-Input Multiple-Output (MIMO) are used to properly processmulti-path for improving the overall system performance. In fact,MIMO-OFDM systems are considered as the technology solution for the IEEE802.11n standard. An example of a MIMO-OFDM system 100 is shown inFIG. 1. A transmitter 102 processes a data stream Tx in an OFDM Txprocessing unit 102 a. This OFDM processing includes sub-carrierallocation and OFDM modulation of each sub-carrier. The modulatedsub-carriers are then mapped to multiple antennas 103 ₁ . . . 103 _(m)according to a MIMO algorithm in a MIMO Tx processing unit 102 b. Oncemapped, the sub-carriers are transmitted to receiver 104 over multipleantennas 103 ₁ . . . 103 _(m) simultaneously.

At receiver 104, the modulated sub-carriers are received on multipleantennas 105 ₁ . . . 105 _(n). A MIMO processing unit 104 a prepares thesub-carriers for demodulation. The sub-carriers are then demodulated inOFDM Rx processing unit 104 b, yielding the Rx data.

One of the challenges of the MIMO-OFDM system design of 802.11n,however, is system capacity. Presently, an efficient method foroptimizing the system capacity of a MIMO-OFDM system does not exist,particularly when the system utilizes a large number of sub-carriers.The “water-pouring” solution, for example, is a technique for increasingsystem capacity by selectively performing power or bit allocation toeach sub-carrier. This technique requires, however, that the transmitterbe aware of channel state information. The transmitter estimates thischannel state information using feedback from a receiver in the system.The signaling overhead of this feedback, however, is significant and canlimit the increase in system performance, particularly in systemstransmitting large amounts of data and/or utilizing a large number ofsub-carriers.

Accordingly, it is desirable to have alternate schemes for optimizingthe system capacity of an MIMO-OFDM.

SUMMARY

The present relates to a method and apparatus for optimizing the systemcapacity of an Orthogonal Frequency Division Multiplexing (OFDM) systemthat uses with Multiple-Input Multiple-Output (MIMO) antennas. In areceiver, a target quality of service (QoS) metric and reference datarate are set. The target QoS metric may be set to a predetermined valueand/or may be adjusted dynamically with respect to packet error rate(PER) by a slow outer-loop control processor. The QoS of receivedsignals are measured and compared to the target QoS. Depending on thecomparison, the receiver generates a channel quality indicator (CQI)which is sent back to the transmitting transmitter. The CQI is a one ortwo bit indicator which indicates to the transmitter to disable, adjustor maintain data transmission rates of particular sub-carriers, groupsof sub-carriers per transmit antenna, or groups of sub-carriers acrossall transmit antennas. At the transmitter, the transmitted data rate isturned-off, increased, decreased, or maintained. At the receiver, thetarget QoS metric and reference data rate are adjusted accordingly. Thisprocess is repeated for each data frame of each sub-carrier group.

BRIEF DESCRIPTION OF THE DRAWINGS

A more detailed understanding of the invention may be had from thefollowing description, given by way of example and to be understood inconjunction with the accompanying drawings wherein:

FIG. 1 illustrates a Multiple-In Multiple-Out (MIMO) OrthogonalFrequency Division Multiplexing (OFDM) system;

FIG. 2 is a flow diagram of a method for optimizing the system capacityof a MIMO-OFDM system;

FIGS. 3A, 3B, and 3C illustrate various sub-carrier groupings; and

FIG. 4 illustrates a MIMO-OFDM system with means for optimizing itssystem capacity utilizing quality measurement bits.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention may be implemented in a WTRU or in a base station.The terminology “WTRU” includes but is not limited to user equipment, amobile station, a fixed or mobile subscriber unit, a pager, or any othertype of device capable of operating in a wireless environment. Theterminology “base station” includes but is not limited to a Node-B, asite controller, an access point or any other type of interfacing devicein a wireless environment.

Elements of the embodiments may be incorporated into an integratedcircuit (IC), multiple ICs, a multitude of interconnecting components,or a combination of interconnecting components and IC(s).

In a preferred embodiment, the system capacity for Orthogonal FrequencyDivision Multiplexing (OFDM) systems that are used with Multiple-InputMultiple-Output (MIMO) antennas is optimized using quality measurements.These quality measurements may be taken continuously, periodically orpreferably, over a sliding window of quality measurement observations.In a MIMO-OFDM receiver, an initial or target quality of service (QoS)metric and corresponding initial reference data rate are set. The QoS ofreceived signals are measured and compared to the target QoS. Dependingon the comparison, the receiver generates one of a plurality of channelquality indicators (CQI) which is sent back to the origin transmitter ofthe signals. The CQI is a one or two bit indicator which informs thetransmitting transmitter to disable, adjust or maintain the datatransmission rates, (i.e., the modulation order of Quadrature AmplitudeModulation (QAM) and channel code rate), of particular sub-carriers orgroups of sub-carriers per transmit antenna. Once the CQI is sent backto the transmitter, the transmitted data rate is disabled, adjusted ormaintained in accordance with the CQI, and at the receiver, the targetQoS metric and the reference data rate are adjusted accordingly. Thisprocess is then repeated for each received signal on each sub-carriergroup, gradually reaching optimal system capacity. This concept isfurther illustrated with reference to FIG. 2.

FIG. 2 illustrates a flow diagram 200 representative of thesystem-optimizing algorithm of the present embodiment. For the purposeof this illustration, signal-to-interference-ratio (SIR) represents theQoS metric of a sample MIMO-OFDM system. It should be understood,however, that any QoS metric, such as for example, signal-to-noise ratio(SNR), bit error rate (BER), and the like, may be utilized in accordancewith the present embodiment to accommodate the needs of a particularuser.

Within receiver 201, an initial target SIR (SIR_(t)) is set (step 202).This target SIR is preferably obtained from a pre-defined storage withinthe receiver 201, such as for example, a look-up table. Alternatively,SIR_(t) the may be obtained and adjusted dynamically with respect topacket error rate (PER) by a slow outer-loop control processor.

In conjunction with setting the SIR_(t) (step 202), an initial referencedata rate (q_(r)) is set (step 204) to a predetermined value. Althoughthe present embodiment describes optimizing the data transmission rateof a MIMO-OFDM system, it should be understood that a MIMO-OFDM systemmay alternatively be optimized in terms of transmission power.

Once the SIR_(t) and q_(r) are set, (steps 202 and 204, respectively),the receiver 201 measures the SIR of the j^(th) frame of a receivedi^(th) sub-carrier group (SIR_(m)) (step 206). A sub-carrier group ispre-defined as a single sub-carrier, as a group of sub-carriers from agiven transmit antenna, or as a group of sub-carriers from multipletransmit antennas. FIGS. 3A-3C illustrates these various sub-carriergroupings. Transmit antennas 302 and 304, for example, each transmitdata over eight sub-carriers, 302 ₁, 302 ₂, . . . 302 ₈, and 304 ₁, 304₂, . . . 304 ₈, respectively. In FIG. 3A, each sub-carrier, 302 ₁-302 ₈and 304 ₁-304 ₈ is pre-defined as a sub-carrier group 306 a-306 pcomprising a single sub-carrier. In FIG. 3B, sub-carriers 302 ₁-302 ₈from antenna 302 are grouped into two sub-carrier groupings, 308 a and308 b. Similarly, sub-carriers 304 ₁-304 ₈ from antenna 304 are groupedinto two sub-carrier groupings, 308 c and 308 d. FIG. 3C illustratessub-carrier groupings 310 a-310 c each comprising sub-carriers from bothantennas 302 and 304.

The measured SIR (SIR_(m)) (step 206) of the i^(th) sub-carrier group isthen compared to the SIR_(t) to calculate their difference according toFormula 1 below:

ΔSIR_(mt(i,j))=SIR_(m(i,j))−SIR_(t(i,j)),   Formula (1)

where i is the respective sub-carrier group number and j is therespective frame number (step 208). The calculated difference betweenthe SIR_(m) and the SIR_(t) (ΔSIR_(mt(i,j))) is then compared to athreshold value (step 210). The threshold value is a pre-defined valuestored in the receiver 201 which represents an acceptable negativevariance from the target SIR. If ΔSIR_(mt(i,j)) yields a negativevariance that is greater than that allowed by the threshold, i.e.,ΔSIR_(mt(i,j)) is less than (−)threshold value, a 2-bit CQI, such as forexample “00”, is generated and sent to the transmitting transmitter (notshown) (step 210 a). This “00” CQI indicates to the transmitter (notshown) to cease transmitting on the current, i^(th) sub-carrier group.

Otherwise, if ΔSIR_(mt(i,j)) is not beyond the pre-defined thresholdlevel, ΔSIR_(mt(i,j)) is compared to the difference between a SIR valueassociated with the transmitted data rate (q) and a SIR value associatedwith the next highest data rate (q30 1) (ΔSIRq_((i,q)) (step 212) inorder to determine whether ΔSIR_(mt(i,j)) is large enough to increasethe current data rate. To make this determination, receiver 201 utilizesa look-up table which represents Data Rate (q) vs. ΔSIRq. This look-uptable is produced from a series of measurements or from simulation(s)and is stored in the receiver 201. In this table, ΔSIRq represents thedifference in SIR between a transmitted data rate q and the next highestdata rate q+1 in the look-up table. Thus, if ΔSIR_(mt(i,j)) is largerthan one-half of ΔSIRq_((i,q)) for a given frame (j) in a givensub-carrier group (i), (i.e., ΔSIR_(mt(j,i))>ΔSIRq_((i,q))/2),ΔSIR_(mt(i,j)) is large enough to increase the data rate (q) to the nexthighest data rate (q+1) in the Data Rate look-up table.

Accordingly, a 2-bit CQI, such as for example “10”, is generated andsent to the transmitting transmitter (not shown) (step 212 a). This “10”CQI indicates to the transmitter (not shown) to increase the currentdata rate (q) to the next highest data rate (q+1) in the Data Rate vs.ΔSIRq look-up table (step 212 b) and to adjust the target SIR_((i,j))(step 212 c) in accordance with Formula 2 below:

SIR_(t(i,j))=SIR_(t(i,j−1))+ΔSIRq_((i,q))/2,   Formula (2)

where SIR_(t(i,j−1)) represents the previous target SIR. Alternatively,the SIR_(t(i,j)) can be adjusted (step 212 c) in accordance with Formula3 below:

SIR_(t(i,j))=SIR_(t(i,j−1))+[ΔSIR_(mt(i,j))−ΔSIR_(mt(i,j−1))].   Formula(3)

If, however, it is determined that ΔSIR_(mt(i,j)) is not greater thanΔSIRq_((i,q))/2 (step 212), ΔSIR_(mt(i,j)) is compared to ΔSIRq_((i,q))(step 214) in order to determine whether ΔSIR_(mt(i,j)) is small enoughto decrease the data rate (q) to the next lowest data rate (q−1) in thelook-up table. To make this determination, receiver 201 utilizes thesame Data Rate vs. ΔSIRq look-up table described above with regards tostep 212. In this comparison, however, if ΔSIR_(mt(i,j)) is less thanone-half of the negative of ΔSIRq_((i,q)), (i.e.,ΔSIR_(mt(i,j))<−(ΔSIRq_((i,q))/2)), a 2-bit CQI, such as for example“01”, is generated and sent to the transmitting transmitter (not shown)(step 214 a). This “01” CQI indicates to the transmitter (not shown) todecrease the data rate (q) to the next lowest data rate (q−1) in theData Rate vs. ΔSIRq look-up table (step 214 b) and to adjust theSIR_(t(i,j)) (step 214 c) in accordance with Formula 4 below:

SIR_(t(i,j))=SIR_(t(i,j−1))−ΔSIRq_((i,q))/2,   Formula (4)

where SIR_(t(i,j−1)) represents the target SIR of the previous dataframe. Alternatively, the target SIR_(t(i,j)) can be adjusted (step 214c) in accordance with Formula 5 below:

SIR_(t(i,j))=SIR_(t(i,j−1))−[ΔSIR_(mt(i,j))−ΔSIR_(mt(i,j−1))].   Formula(5)

It should be understood that the difference between successive datarates, (i.e., step size), in the data rate table of steps 212 and 214does not necessarily have to be uniform. In fact, it may be variedaccording to a user's needs. For example, the step size in the data ratetable may be four (4) for the first x-number of frames (in transientstate), while the step size for all frames after the xth frame can beone (1) (steady state).

After comparing the difference between the SIR_(m) and the SIR_(t) for agiven frame (j) in a given sub-carrier group (i) (ΔSIR_(mt(i,j))) to thethreshold value in step 210 and to ΔSIRq_((i,q)) in steps 212-214, it isdetermined if ΔSIR_(mt(i,j)) is within the threshold value (step 210)and neither large enough to increase the current data rate (step 212)nor small enough to decrease the current data rate (step 214). IfΔSIR_(mt(i,j)) meets that criteria, a 2-bit CQI, such as for example“11”, is generated and sent to the transmitting transmitter (not shown)(step 216). This “11” CQI indicates to the transmitter (not shown) tocontinue transmitting at the current data rate.

It should be noted that steps 206 through 216 of this process 200comprise a looping algorithm which is repeated for all sub-carriergroups (i) and for all frames (j). In addition, the target SIR_(t(i,j))and reference data rate _((i,j)) of a given sub-carrier group (i) andframe (j) act as the reference SIR_(t) and reference data rate (q_(r)),respectively, for the next frame (j+1) in the i^(th) sub-carrier group.It is this continual updating of the transmitted data rate that causesthe MIMO-OFDM system to gradually reach its optimal performance level.

A MIMO-OFDM system 400 with means for optimizing its system capacityutilizing quality measurement bits in a manner described herein is shownin

FIG. 4. A transmitter 402 processes a data stream Tx in an OFDMprocessing unit 402 a. This OFDM processing includes sub-carrierallocation and OFDM modulation of each sub-carrier. The modulatedsub-carriers are then mapped to multiple antennas 403 ₁ . . . 403 _(m)according to a MIMO algorithm in a MIMO Tx processing unit 402 b. Oncemapped, the sub-carriers are transmitted to receiver 404 over multipleantennas 403 ₁ . . . 403 _(m) simultaneously.

At the receiver 404, the modulated sub-carriers are received on multipleantennas 405 ₁ . . . 405 _(n). The received sub-carriers are sent to aMIMO Rx processing unit 404 a where an inverse MIMO algorithm preparesthe sub-carriers for demodulation. The MIMO decoded sub-carriers arethen sent to OFDM Rx unit 404 b where they are demodulated. Next, thedemodulated data is sent to a Channel Quality Measurement unit 404 c,frame by frame, wherein a quality measurement is taken for each dataframe. Each of these quality measurements are then sequentially comparedto a target quality metric in a Channel Quality Comparison unit 404 d.Depending on the comparison, a Channel Quality Indicator (CQI) Signalingunit 404 e generates a one or two bit CQI for each measured data frameand sends the CQIs to a MUX unit 404 f for processing. These CQIs arethen modulated in an OFDM Tx unit 404 g, and mapped to multiple antennas405 ₁ . . . 405 _(n), via MIMO Tx unit 404 h for transmission totransmitter 402.

At the transmitter 402, the encoded CQIs are received on multipleantennas 403 ₁ . . . 403 _(m), prepared for demodulated in MIMO Rx unit402 c, and demodulated in OFDM Rx unit 402 d. Once demodulated, theextracted data is sent to a CQI recovery unit 402 e where the one or twobit CQI is extracted and processed. The OFDM processing unit 402 a thenallocates and modulates the sub-carriers with the next Tx data streamaccording to the processed CQI information. This entire process is thenrepeated so as to iteratively increase (or decrease) the datatransmission rate of a given sub-carrier thereby maximizing the system'scapacity.

In an alternate embodiment, the CQI can be sent as a 1-bit indicator,where one state of the binary bit would indicate to the transmitter toincrease the data rate to a higher level and the other state of thebinary bit is sent to indicate to the transmitter to decrease thetransmitted data rate.

Although the features and elements of the present invention aredescribed in the preferred embodiments in particular combinations, eachfeature or element can be used alone without the other features andelements of the preferred embodiments or in various combinations with orwithout other features and elements of the present invention. Further,the features and elements of the present invention may be implemented ona single IC, such as an application specific integrated circuit (ASIC),multiple ICs, discrete components, or a combination of discretecomponents and ICs. Moreover, the present invention may be implementedin any type of wireless communication system. In some deployments, theIC(s)/discrete components may have some of these features and elements,which are totally or partially disabled or deactivated.

While the present invention has been described in terms of the preferredembodiment, other variations which are within the scope of the inventionas outlined in the claims below will be apparent to those skilled in theart.

1. A wireless transmit/receive unit (WTRU) comprising: at least oneprocessor configured to receive an orthogonal division multiple access(OFDM) signal; wherein the OFDM signal includes a plurality ofsubcarriers; wherein the at least one processor is configured to performa quality measurement for each of a plurality of groups of thesubcarriers; wherein the at least one processor is configured todetermine a first value associated with a channel quality indicator(CQI) table; wherein the first value indicates a data rate; wherein theat least one processor is configured to determine for each of theplurality of groups, an n-bit indication of a difference in a channelquality indicator (CQI) table between the first value and a valueassociated with that group in response to the quality measurement ofthat group; wherein the at least one processor is further configured totransmit the n-bit indications.
 2. The WTRU of claim 1 wherein the n-bitindications are 2-bit indications.
 3. The WTRU of claim 1 wherein eachof the n-bit indications is associated with all transmit antennas of abase station.
 4. The WTRU of claim 1 wherein the at least one processoris configured to receive a MIMO OFDM signal; wherein the MIMO OFDMsignal includes a plurality of MIMO signals; wherein each of the n-bitindications is associated with one of the plurality of MIMO signals andone of the groups of subcarriers.
 5. The WTRU of claim 1 wherein thequality measurements are signal to interference ratios.
 6. A methodcomprising: receiving, by wireless transmit/receive unit (WTRU), anorthogonal division multiple access (OFDM) signal; wherein the OFDMsignal includes a plurality of subcarriers; performing a qualitymeasurement for each of a plurality of groups of the subcarriers;determining a first value associated with a channel quality indicator(CQI) table; wherein the first value indicates a data rate; determiningfor each of the plurality of groups, an n-bit indication of a differencein a channel quality indicator (CQI) table between the first value and avalue associated with that group in response to the quality measurementof that group; wherein the at least one processor is further configuredto transmit the n-bit indications.
 7. The method of claim 6 wherein then-bit indications are 2-bit indications.
 8. The method of claim 6wherein each of the n-bit indications is associated with all transmitantennas of a base station.
 9. The method of claim 6 wherein the OFDMsignal is a MIMO OFDM signal; wherein the MIMO OFDM signal includes aplurality of MIMO signals; wherein each of the n-bit indications isassociated with one of the plurality of MIMO signals and one of thegroups of subcarriers.
 10. The method of claim 6 wherein the qualitymeasurements are signal to interference ratios.
 11. A base stationcomprising: at least one processor configured to transmit an orthogonaldivision multiple access (OFDM) signal; wherein the OFDM signal includesa plurality of subcarriers; wherein the at least one processor isconfigured to receive a plurality of n-bit indications; wherein each ofthe n-bit indications corresponds to a respective group of thesubcarriers; wherein each of the n-bit indications indicates adifference in a channel quality indicator (CQI) table between a firstvalue and a value associated with the respective group; wherein the atleast one processor is further configured to transmit a subsequent OFDMsignal formatted in response to the received n-bit indications.
 12. Thebase station of claim 11 wherein the n-bit indications are 2-bitindications.
 13. The base station of claim 11 wherein each of the n-bitindications is associated with all transmit antennas of the basestation.
 14. The base station of claim 11 wherein the OFDM signal is aMIMO OFDM signal; wherein the MIMO OFDM signal includes a plurality ofMIMO signals; wherein each of the n-bit indications is associated withone of the plurality of MIMO signals and one of the groups ofsubcarriers.
 15. The base station of claim 11 wherein the qualitymeasurements are signal to interference ratios.